[quicwg/base-drafts] Does minimal encoding apply to extensions? (#3402)

[Dmitri Tikhonov <[e-mail]>]

[Section 12.4 of the Transport Draft](draft-ietf-quic-transport-25#section-12.4) restricts frame type varint encoding:

```
   The Frame Type field uses a variable length integer encoding (see
   Section 16) with one exception.  To ensure simple and efficient
   implementations of frame parsing, a frame type MUST use the shortest
   possible encoding.  Though a two-, four- or eight-byte encoding of
   the frame types defined in this document is possible, the Frame Type
   field for these frames is encoded on a single byte.  For instance,
   though 0x4001 is a legitimate two-byte encoding for a variable-length
   integer with a value of 1, PING frames are always encoded as a single
   byte with the value 0x01.  An endpoint MAY treat the receipt of a
   frame type that uses a longer encoding than necessary as a connection
   error of type PROTOCOL_VIOLATION.
```

The relevant sentence is:

> Though a two-, four- or eight-byte encoding of the frame types defined in this document is possible, the Frame Type field for these frames is encoded on a single byte.

This means that the restriction may not apply to frames defined by QUIC extensions (such as the `ACK_FREQUENCY` frame in the _Delayed Acks_ extension).  Must each extension spell out its own frame type encoding rules?

It would be easier to change the transport draft to extend the minimal encoding restriction to not-as-yet-specified frames as well.

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